It is known to provide a motor vehicle with a lean NOx trap (LNT) aftertreatment system to control the emission of NOx to the environment. It is further known to provide a motor vehicle with an electric machine connected to the engine such as an integrated starter generator that can be used to provide a short torque boost to the engine during vehicle acceleration events and stop start events. Such a vehicle is often referred to as being a Mild Hybrid Electric Vehicle (MHEV) because electrical power can be used to assist the engine but is not used to power the vehicle on its own.
It is further known that during use the LNT will gradually become contaminated with sulfur which is a by-product of combustion. The effect of such sulfur contamination is that active sites within the LNT are reduced and can no longer trap NOx as the contamination increases. Ultimately, such contamination may result in the vehicle to which the LNT is fitted no longer being compliant with emissions standards.
When a vehicle is identified as an emissions failure it is required to be taken to a service provider (garage) to be repaired. However, the service provider will not normally have the ability to provide sufficient load to the engine when the vehicle is in the workshop to remove the sulfur from the LNT. This is because to remove the sulfur requires the engine to be run rich of stoichiometric, which is achievable when a high torque demand is being met by the engine.
One solution to this problem is for the service provider to drive the vehicle on a defined drive cycle to provide sufficient torque to enable a rich exhaust gas to be produced. However, this is often problematic as traffic, road signals and other uncontrolled events can prevent the drive cycle being successfully completed thereby resulting in a failure to remove all the sulfur from the LNT.
It is an object of the present invention to provide a method of removing sulfur from a lean NOx trap that overcomes the problems associated with the prior art.